The invention relates to ion implantation apparatus. Such an apparatus is used for implanting ions of preselected chemical species into semiconductor wafers for manufacturing desired semiconductor electronic devices. The present invention is particularly concerned with apparatus for scanning semiconductor wafers under treatment relative to the ion beam to ensure an even dose of ions is delivered to all parts of the wafer.
In one form of ion implantation apparatus, wafers for treatment are mounted at a number of discreet positions around the periphery of an implant wheel. The wheel is mounted at the free end of a scanning arm which is mounted at its other end for reciprocating motion about a scanning axis. The scanning arm and implant wheel are located in a vacuum chamber to which a beam of desired ions is directed from an ion source. As the wheel rotates about a wheel axis at the end of the scanning arm, the various wafers mounted around the periphery of the wheel are brought successively in front of the beam. At the same time, the scanning arm more slowly moves the axis of rotation of the wheel to and fro, so that the ion beam is progressively scanned over the whole surface of the wafers.
Apparatus of this kind is known from U.S. Pat Nos. 4,733,091 and U.S. Pat No. 5,641,969.
In U.S. Pat No. 4,733,091, the scanning arm of the apparatus is mounted for reciprocating movement by means of a rotary vacuum seal on the scan axis of the scanning arm. The implant wheel is driven by means of drive shafts and belts from outside thee vacuum chamber, with the drive belts extending the length of the scanning arm to drive the shaft of the implant wheel rotatably mounted at the free end of the scanning arm.
An improvement of this apparatus is disclosed in U.S. Pat No. 5,641,969. In this patent a motor is mounted at the free end of the scanning arm so as to rotate directly the implant wheel about the wheel axis. The scan arm is driven by a separate motor mounted outside the vacuum chamber through a rotary vacuum seal.
In this prior art apparatus the scan arm is driven by a leadscrew, arranged such that the velocity of the leadscrew is proportional to the velocity of the centre of the wheel relative to the beam. Custom electronics then provide a control velocity to the leadscrew that varies as a function of wheel radius in the beam. This arrangement provides very high levels of process uniformity. In this arrangement the whole scanning system is mounted at an angle relative to the ion beam of typically 70, such that the ion beam is perpendicular to a wafer mounted on the wheel which is canted inwardly 70 towards the axis of the wheel. This mounting causes motion of the wafer along the line of the ion beam as the wheel is scanned. Because the beam is not a perfectly parallel beam, this movement causes non-uniformities in the implantation operation.
According to a first aspect of the present invention there is provided an ion implantation apparatus comprising a vacuum chamber, an ion beam generator to generate an ion beam in the vacuum chamber, an implant wheel in the vacuum chamber, having a plurality of circumferentially distributed wafer support elements each having a wafer support surface canted inwardly towards the centre of the implant wheel, a scanning arm mounted for reciprocal movement about a scan axis and having a free end supporting the implant wheel for rotation about a wheel axis, so that rotation of the implant wheel about the wheel axis brings the wafer support elements successively to intercept the ion beam and reciprocation of the scanning arm about the scan axis scans the ion beam across the wafer support elements, wherein the wheel is rotatable with respect to the arm about a tilt axis, which substantially intersects the wheel axis and ion beam, to vary the direction of implantation, and wherein the scan axis is substantially parallel to the direction of the ion beam, and the direction of the wheel axis is offset from the direction of the scan axis such that the ion beam is substantially perpendicular to the wafer support surfaces when the implant wheel is not tilted.
This arrangement ensures that the wafers are always kept at a constant distance with respect to the ion beam, thereby achieving uniform implantation. Even when the wheel is tilted to vary the implantation angle, the position of the tilt axis ensures that there is little, if any, variation in the distance of the wafers with respect to the ion beam.
The tilt axis is preferably substantially perpendicular to the wheel axis as this provides a simple construction with little movement of the substrate relative to the ion beam when the wheel is tilted. However, if even this little movement is not tolerable, the tilt axis can substantially pass through the plane of the substrate support for the substrate being scanned by the ion beam thereby eliminating any relative movement between the substrate and the ion beam. Any positions between these two particular examples will also give favourable results.
With the arrangement of the present invention, as the scan axis and wheel axis are no longer parallel, the linear relationship between the scanning position of the wafer and the leadscrew is lost.
Therefore, according a related aspect of the present invention there is provided ion implantation apparatus comprising a vacuum chamber, an ion beam generator to generate an ion beam in the vacuum chamber, an implant wheel in the vacuum chamber, having a plurality of circumferentially distributed wafer support elements, a scanning arm mounted for reciprocal movement about a scan axis and having a free end supporting the implant wheel for rotation about a wheel axis, so that rotation of the implant wheel about the wheel axis brings the wafer support elements successively to intercept the ion beam and reciprocation of the scanning arm about the scan axis scans the ion beam, across the wafer support elements, and a motor for driving the scanning arm, the motor having a drive shaft connected through a gearbox which has an output shaft driving the scanning arm, wherein a first rotary position detector is provided on the drive shaft and a second rotary position detector is provided on the output shaft.
This arrangement provides an accurate way of determining the scanning position of the wafers. The use of the rotary position detectors either side of the gearbox allows a very accurate measurement to be made as the drive shaft is rotating faster than the output shaft by an amount equal to the ratio of the gears, which can be typically 170:1.
The data produced by the rotary position detectors can be used to provide full positional information relating to the scan arm, and this can therefore be used for safety circuits and other related equipment thereby eliminating the need for the optical sensors provided in the prior art.
The use of the two rotary position detectors allows the mechanical integrity of the apparatus to be readily verified using a method comprising the steps of taking a reading from the second rotary position detector, calculating the theoretical position of the motor from the reading taken from the second rotary position detector and from the gearbox ratio, measuring the actual position of the motor from the first rotary position detector, and comparing the actual position of the motor with the theoretical position of the motor.
Further, the velocity profile of the apparatus can easily be corrected to allow for rotary position detector non-linearity. In this case a method is used comprising the steps of driving the scan arm across a range of movement, and noting readings from both rotary position detectors in each position, calculating from each first rotary position detector reading and from the gearbox ratio the theoretical second rotary position detector reading in each position, subtracting the theoretical second rotary position detector reading from the actual second rotary position detector reading in each position, filtering the data generated in order to remove errors generated by sources other than second rotary position detector non-linearity to obtain data indicative only of second rotary position detector non-linearity, differentiating this data to produce data indicative of velocity error, and applying this velocity error to a look up table of the desired velocity profile.
According to a further aspect of the present invention, there is provided a method of driving a member through a pre-determined velocity profile, the method comprising compiling numerical tables for velocity and position for the velocity profile, using values from the numerical tables instructing the member to move from a first position at which it is travelling at a first velocity to a second position at which it is travelling at a second velocity at an acceleration at which the member will reach the second velocity before it reaches the second position and will hence stop accelerating until it reaches the second position, and selecting the rate of reading values from the numerical tables to be fast enough that the inherent lag in the control system causes smooth acceleration of the arm throughout the velocity profile.
In this case, the member to be driven may be the scan arm referred to above.
According to a further aspect of the present invention, there is provided an ion implantation apparatus comprising a vacuum chamber, an ion beam generator to generate an ion beam in the vacuum chamber, an implant wheel in the vacuum chamber, having a plurality of circumferentially distributed wafer support elements, a scanning arm mounted for reciprocal movement about a scan axis and having a free end supporting the implant wheel for rotation about a wheel axis, so that rotation of the implant wheel about said wheel axis brings the wafer support elements successively to intercept the ion beam and reciprocation of the scanning arm about the scan axis scans the ion beam across the wafer support elements, and a motor for driving the scanning arm, the motor having a drive shaft connected directly to a cycloid type gearbox, the output of which directly drives the scanning arm.
The scan arm requires only a relatively low power motor and low torque gearbox to drive the scan arm. The use of a cycloid type gearbox, which is capable of generating very large torque may seem a surprising choice. However, such an arrangement is beneficial as it is capable of resisting very high reserve torque in case of failure.